In a known technology of producing a single crystal, a seed crystal is brought into contact with a melt of a heated raw material, and this seed crystal is pulled up, so that a single crystal is produced. In general, the formation of the single crystal is performed by the following procedure. A crucible holding a raw material of a single crystal and provided with a surrounding heating device is disposed in the inside of a furnace, the crucible is heated by the above-described heating device, and a melt of the raw material is heated by the crucible heated to a high temperature. When a seed crystal having a diameter smaller than the diameter of the single crystal to be produced is brought into contact with the surface of this melt, the melt brought into contact with the seed crystal dissipates heat through the seed crystal and is cooled, so that a crystal grows while being aligned in the direction of the crystal of the seed crystal. The seed crystal is pulled up in accordance with the growth of the crystal, the grown crystal is cooled sequentially and, therefore, the crystal further grows. This is repeated, and the single crystal is produced. In the initial stage of growth of the single crystal, in order to increase the diameter of the crystal, a taper portion having a diameter becoming conically increased from the seed crystal is formed while the seed crystal is pulled up. When the diameter of the seed crystal reaches a predetermined size, the formation of the taper portion having a diameter becoming gradually increased is terminated, and the formation of the straight body portion is started, wherein the crystal grows cylindrically in the axis direction.
At this time, since the liquid level of the melt is positioned below the upper end of the crucible, the single crystal being pulled up is heated by application of the radiation heat from the inner perimeter of the crucible while the crystal is positioned below the upper end of the crucible, and a portion of the single crystal positioned higher than the upper end of the crucible dissipates heat and is cooled. After an adequate length of straight body portion is formed, the straight body portion is isolated from the melt, and the amount of heating of the crucible is reduced gradually and the crucible is cooled to room temperature.
In such a technology of producing a single crystal, when defects, cracks, and the like occur, portions including the defects, cracks, and the like become defective portions, so that no product is available. Therefore, it is necessary for an improvement of the productivity to reduce the occurrence rate of defective portions, i.e. the fraction defective. One of the causes of occurrence of defects, cracks, and the like in the single crystal is attributed to a temperature distribution in the single crystal during the formation of the single crystal and during the cooling of the formed single crystal.
With respect to a technology of producing a single crystal to appropriately adjust such a temperature distribution, i.e. temperature gradient, in the single crystal in a longitudinal direction during the formation of the single crystal and during the cooling of the single crystal, Japanese Unexamined Patent Application Publication No. 8-175896 proposes that a radiation heat reflector is disposed above a single crystal so as to suppress the heat dissipation toward an upper portion above the single crystal, and the radiation heat reflector is moved upward as the single crystal is pulled up. Japanese Unexamined Patent Application Publication No. 6-157187 proposes that a movable refractory lid is disposed on the upper opening of a furnace containing a crucible for producing a single crystal. In addition, Japanese Unexamined Patent Application Publication No. 2001-316195 proposes that a lid is disposed on the upper portion of a crucible for producing a single crystal.
On the other hand, natural convection caused by a density gradient resulting from a temperature gradient of a melt in a crucible, Marangoni convection caused by a surface tension gradient, and the like occur in the melt in the crucible. When these types of convection of the melt occur, a portion at which the single crystal is formed, i.e. a solid-liquid interface portion of the single crystal, does not become flat, but becomes concave or convex, for example. Consequently, a temperature distribution in the radius direction in the vicinity of the solid-liquid interface portion of the single crystal becomes nonuniform. With respect to a technology of producing a single crystal to reduce such nonuniformity of the temperature distribution in the vicinity of the solid-liquid interface portion of the single crystal, Japanese Unexamined Patent Application Publication No. 6-183877 proposes that when a single crystal is pulled up, the single crystal is rotated or a cylindrical crucible is rotated in a rotational axis which is a central axis of a cylindrical crucible, forced convection to spread the melt in the radius direction of the crucible is effected and, thereby, a convection pattern of the entire melt is controlled in order to flatten the shape of the solid-liquid interface portion of the single crystal.
In Japanese Unexamined Patent Application Publication No. 8-175896, a temperature distribution in the single crystal in a longitudinal direction is reduced in the process of forming the single crystal in order to reduce the occurrence of defects, cracks, and the like. However, when a radiation heat reflector is disposed at the position immediately above the single crystal, the difference in the temperature distribution in the single crystal in the radius direction during cooling is increased and, therefore, it is difficult to reduce the occurrence of defects, cracks, and the like. In Japanese Unexamined Patent Application Publication No. 6-157187, the lid is disposed on the upper portion of refractory in a cooling process in order to reduce the cooling rate and to reduce the temperature distribution in the single crystal and, thereby, to reduce the occurrence of defects, cracks, and the like. However, the control of the surface temperature of the single crystal in the process of forming the single crystal is not taken into consideration, so that the temperature distribution cannot be made appropriate in the process of forming the single crystal, and it is difficult to reduce the occurrence of defects, cracks, and the like as well.
In Japanese Unexamined Patent Application Publication No. 2001-316195, since the lid is disposed on the upper portion of the crucible, the surface temperature of the single crystal in an upper portion above the crucible cannot be controlled in the cooling process, the temperature distribution cannot be made appropriate in the process of forming the single crystal as well, and it is difficult to reduce the occurrence of defects and cracks. In addition, since the lid is disposed on the upper portion of the crucible in the initial stage of the process of forming the single crystal as well, the temperature of the surface of the taper portion of the single crystal, i.e. the taper surface, is raised, the surface is roughened and, contrary to expectations, defects, cracks, and the like may tend to occur. Consequently, these technologies of producing a single crystal may not reduce the occurrence of defects, cracks, and the like, and it is difficult to reduce the fraction defective.
On the other hand, in high-frequency induction heating by the use of a high-frequency generation device including a high-frequency coil as a heating device, the heating value per unit volume in a unit time is proportional to the square of the current Jθ in the circumferential direction, the current being induced on the surface of the metal. The Jθ increases as the metal is located closer to the coil. The electromagnetic field has a property of concentrating on the surface of the corner portion of the metal. Therefore, when the cylindrical crucible is heated by the high-frequency induction heating, the electromagnetic field concentrates on the upper end portion of the side wall of the crucible and the periphery of the bottom serving as the lower end portion. Consequently, the heating values at the upper end portion and the lower end portion of the side wall of the crucible become larger than those of other portions. In this manner, for example, as the side wall becomes longer in the axis direction than the diameter of the bottom, the temperatures of the portions of the side wall other than the upper end portion and the lower end portion of the side wall of the crucible become lower than the temperatures of the upper end portion and the lower end portion. Therefore, the temperature distribution of the crucible becomes nonuniform, and the melt exhibits an undesirable convection pattern even when the seed crystal and/or the crucible are rotated as disclosed in Japanese Unexamined Patent Application Publication No. 6-183877, so that the solid-liquid interface portion of the single crystal may not be flattened.
Furthermore, even when the solid-liquid interface portion of the single crystal can be flattened by effecting the forced convection to spread in the radius direction through the rotation of the single crystal and/or the crucible, as disclosed in Japanese Unexamined Patent Application Publication No. 6-183877, in the process of forming the taper portion from the seed crystal, since the outer diameter of the crystal is small compared with that in the formation of the straight body portion of the single crystal, the forced convection to spread in the radius direction cannot be effected adequately. Consequently, in the stage of formation of the taper portion from the seed crystal, the melt in the crucible may exhibits an undesirable convection pattern, so that the solid-liquid interface portion of the single crystal may not be flattened, depending on the conditions of the size of the bottom of the crucible and the like. As described above, even in the technology of producing a single crystal disclosed in Japanese Unexamined Patent Application Publication No. 6-183877, since the solid-liquid interface portion of the single crystal is not flattened and the temperature distribution becomes nonuniform depending on the conditions of the size and the shape of the crucible and the like, the occurrence of defects, cracks, and the like may not be reduced, and it is difficult to reduce the fraction defective.